Characterization of intrinsic factors that determine GABAA receptor (GABAR) inhibitory postsynaptic current (IPSC) shape and the magnitude of extrasynaptic tonic inhibitory current are fundamental issues in inhibitory synaptic physiology. Alterations in phasic, synaptic (? subunit-containing) and tonic, extrasynaptic (d subunit-containing) inhibition due to mutations of ? and d subunits have been shown to underlie several types of genetic generalized epilepsies. To understand the bases for genetic epilepsies involving GABAR gene mutations, it is necessary to understand the functional properties of GABARs that mediate phasic and tonic inhibition. The goals of this proposal are to determine the GABAR functional properties and structural domains that regulate phasic and tonic GABAR currents and to characterize the functional deficits in phasic and tonic GABAerqic inhibition produced by ?2S and d subunit epilepsy mutations. The hypotheses to be tested are that: a) The structural determinants of aa? and aad GABAR gating modes involve non-channel (non-M2) structures including M2/M3, M1 and the N-terminus loop 2. b) Fast/intermediate and slow/ultraslow desensitization of GABAR currents are determined by distinct, structurally separate domains, c) The ?2L(R42Q) mutation reduces "synaptic" a1a2?2L(R42Q) current by reducing cell surface expression, d) The ?2L(K289M) mutation reduces "synaptic" a1a2?2L (K289M) current by destabilizing channel open states, e) The ?2L(Q351X) mutation reduces "synaptic" a1a2?2L (Q351X) current by impairing receptor assembly and decreasing surface expression, f) The d(mutant) reduces "tonic" a4a2d(mutant) current by destabilizing channel open states. The specific aims are to determine: a) The structural determinants of aa? and aad GABAR gating modes, b) The structural determinants of desensitization of aa? and aad GABARs. c) The basis for the reduced "synaptic" current in heterozygous a1a2?2L(R42Q) GABARs. d) The basis for the reduced "synaptic" current in heterozygous a1a2?2L(K289M) GABARs. e) The basis for the reduced "synaptic" current in heterozygous a1a2?2L(Q351X) GABARs. f) The basis for the reduced "synaptic" current in heterozygous a4a2d(mutant) GABARs.